On universal theories of defaults

Though unifications of some of the numerous theories of default reasoning have been found, we add to doubts about the existence of universal theories by viewing default reasoning from the standpoint of decision theory as a case of rational self-government of inference. Default rules express not only methods for deriving new conclusions from old, but also preferences among sets of possible conclusions. Conflicting default rules, which form the central difficulty in the theories, represent inconsistent preferences about conclusions. These conflicting rules cannot be avoided, as they arise naturally in practice, especially in databases representing the knowledge of several experts. We compare these theories of rational inference with theories of group decision making, and develop doubts about universal theories of the former by considering well-known negative results about the latter.